Phosphonium-based room temperature ionic liquids (“RTILs”) are molten salts with a melting temperature below 100° C. (see, for example, Del Sesto et al, “Tetraalkylphosphonium-based Ionic Liquids,” J. Organomet. Chem., 2005, vol. 690, pp. 2536-2542, and Fraser et al., “Phosphonium-Based Ionic Liquids: An Overview,” Aust. J. Chem., 2009, vol. 62, pp. 309-321, both incorporated by reference herein). They resist oxidation and reduction over a wide voltage range. They are recyclable. They are typically less dense than water. They tend to be more thermally stable and less expensive than nitrogen-based RTILs. Many are commercially available. Phosphonium-based RTILs have been used for a variety of applications including phase transfer catalysis, hydroformylation, carbonylation, liquid-liquid extraction, and some aspects of mass spectrometry. Little is known about phosphonium-based RTILs as matrices in matrix-assisted laser desorption ionization (“MALDI”) mass spectrometry.